April 2, 2013

Rails Basics: Intro to has_many :through

Okay here we go. Time to learn about the Rails has_many :through association. Below is a walkthrough I created based on my early efforts to get a grasp of this concept… for a more thorough explanation, check out the official Rails guide or Treehouse’s intro.

1. Create your app

rails new sb8
cd sb8

2. Generate scaffolding

We’ll need a model for Physicians, Patients, and Appointments. Create them as follows:

rails generate scaffold Physician name:string
rails generate scaffold Patient name:string
rails generate scaffold Appointment physician_id:integer patient_id:integer appointment_date:datetime

Finally, migrate the database.asdasdasd

rake db:migrate

3. Setup the model associations

Make your model/appointment.rb file look like this:

class Appointment < ActiveRecord::Base
  attr_accessible :appointment_date, :patient_id, :physician_id
  belongs_to :physician
  belongs_to :patient

Make your model/physician.rb file look like this:

class Physician < ActiveRecord::Base
  attr_accessible :name
  has_many :appointments
  has_many :patients, :through => :appointments

Make your model/patient.rb file look like this:

class Patient < ActiveRecord::Base
  attr_accessible :name
  has_many :appointments
  has_many :physicians, :through => :appointments

4. Create some records

Now, you can go to the URLs below and create records for each of the models. Create some Physicians and some Patients first. For the Appointments, simply enter the ID number of the respective Physicians and Patients. The URLs are:


5. View the associations via console

Once you do this, you can view the associations via console. First go into console by typing:

rails c

Now, if you set a Patient into a variable like this:

p = Patient.find(1)

…You can then view all of that Patient’s appointments or physicians by typing this:


The commands above let me view a patient’s appointments, and a patient’s physicians. But wait, there’s more – I can take this a step further. Suppose I want to view a patient’s physician’s appointments, OR a patient’s appointment’s physician. You can do these as follows:

# Set a patient into the variable 'p'
p = Patient.find(1)

# Show the patient's physician for a certain appointment

# Show the appointments belonging to one of the patient's physicians

The main point here is when models are associated, you can use jump up/down the chain in a single line of code – and Ruby is able to understand what you’re looking for.